Studies are proposed in experimental animals and man that are designed to characterize the underlying platelet involvement in atherogenesis, wound healing, arterial thrombosis and hemostasis and to define the therapeutic role of inhibiting platelet function with pharmacologic agents. Studies in baboons will focus on: 1) mechanisms of formation and prevention of experimental atherosclerosis induced by vascular injury using homocystinemia, mechanical deendotherlialization, immune injury or infectious vascular damage. The effects of altering plasma lipid levels and administering platelet inhibitory agents will be evaluated; 2) in vivo evaluation of antithrombotic therapy of pharmacologic inhibitors of platelet function by assessing both platelet release and consumption in vivo using measurements of 5lCr-and l4C-serotonin platelet survival; 3) examination of the role of platelets in wound healing by in vitro and in vivo alterations in platelet number or function. In patients the hemostatic mechanism will be evaluated kinetically using 5lCr-platelets, l3lI-fibrinogen and l25I-plasminogen to clarify: l) complex bleeding disorders (e.g. hemorrhagic diathesis of liver disease; consumptive coagulopathy of surgery and trauma; miscellaneous undiagnosed bleeding disorders); 2) arterial thrombotic disease (e.g., recurrent arterial and venous thromboembolism, coronary artery disease, hyperlipidemia, transient ischemic attacks); and 3) evaluation of the significance of platelet size measurements.